A speed ratio of an automobile is governed by a mechanism for transmitting torque from a prime mover such as an engine to drive wheels. In general, a gear stage of a vehicle transmission including a forward stage and a reverse stage is selected by selecting a gear pair for transmitting the torque from a plurality of gear pairs by a clutch. Given that the transmission is formed by combining a plurality of planetary gear units, a torque transmission route is changed depending on engagement states of a clutch and a brake. Alternatively, the gear stage of the transmission may also be shifted by changing engagement states of the clutch and the brake thereby changing a reaction element or an output element of the planetary gear unit. In the transmissions of those kinds, the gear stage including the forward stage and the revers stage can be changed by disengaging the engagement device engaged to establish the current gear stage while engaging another engagement device to establish another gear stage. Such method is called “the clutch-to-clutch shifting”. However, if torque transmitting capacities of both engagement devices to be engaged and disengaged are insufficient during the clutch-to-clutch shifting, so-called a “retraction” feeling may be caused by a torque drop. By contrast, if torque transmitting capacities of both engagement devices to be engaged and disengaged are excessive, shocks may be caused by a torque reduction resulting from a transmission tie-up.
Therefore, the torque transmitting capacities of the engagement devices to be engaged and disengaged have to be controlled in just proportion. Various kinds of control systems and methods have been proposed for this application. For example, Japanese Patent Laid-Open No. 2008-051186 describes a shift control system. According to the teachings of Japanese Patent Laid-Open No. 2008-051186, a torque of one of clutches involved in the clutch-to-clutch shifting is determined based on a torque sharing rate between a target output torque of a transmission and a torque of said one of the clutches, and a torque of the other clutch is determined based on the torque of said one of the clutches and the target output torque. In addition, the torque of the clutch to be disengaged is reduced gradually in a manner such that the torque becomes zero at a commencement of an inertia phase. Accordingly, the torque will not be applied to the clutch to be disengaged during the inertia phase, and the shifting operation is progressed by increasing hydraulic pressure applied to the clutch to be engaged.
Japanese Patent Laid-Open No. 2001-227637 also describes a control device for automatic transmission configured to change a torque transmitting capacity of the clutch to be engaged in accordance with a change in an input speed during a torque phase of the clutch-to-clutch shifting. If the torque transmitting capacity of the clutch to be engaged is insufficient during the clutch-to-clutch shifting, an engine speed or an input speed is increased while the power is on. By contrast, if the torque transmitting capacity of the clutch to be engaged is excessive, the “retraction” feeling may be caused by a torque drop resulting from the transmission tie-up. In order to avoid such disadvantages, according to the teachings of Japanese Patent Laid-Open No. 2001-227637, the torque transmitting capacity of the clutch to be engaged is increased in case the input speed is increased, and the torque transmitting capacity of the clutch to be engaged is decreased in case the input speed is lowered.
As described, according to the teachings of Japanese Patent Laid-Open No. 2008-051186, the torque transmitting capacity of the clutch to be disengaged or the hydraulic pressure applied thereto is reduced to zero at the commencement of the inertia phase. Therefore, if the torque transmitting capacity of the clutch to be disengaged or the hydraulic pressure applied thereto is increased to a certain extent just before the commencement of the inertia phase, the clutch is disengaged abruptly at the commencement of the inertia phase. In this situation, the torque transmitting capacity of the clutch to be engaged or the hydraulic pressure applied thereto has to be increased abruptly to make up for the reduction in the torque transmitting capacity of the disengaged clutch. Thus, the torque transmitting capacity of the clutch or the hydraulic pressure applied thereto has to be controlled irregularly thereby causing shift shocks.
As also described, the control device taught by Japanese Patent Laid-Open No. 2001-227637 is configured to control the clutch to be engaged during the torque phase of the clutch-to-clutch shifting. According to the teachings of Japanese Patent Laid-Open No. 2001-227637, rotational speeds of rotary members including the engine are changed positively. Therefore, the teachings of Japanese Patent Laid-Open No. 2001-227637 may not be applied to control entire course of the clutch-to-clutch shifting.